The present invention relates to a method of manufacturing a piezoelectric composite substrate structure. More particularly, the invention relates to joining, by wafer bonding, a piezoelectric material and another substrate. The present invention is further directed to a structure comprising a piezoelectric composite substrate.
Piezoelectric materials have been widely used as component materials for electronic devices used in communication equipment, data processing equipment, or other similar equipment. Various piezoelectric materials have also been used as component materials for communication devices. In particular, single crystal piezoelectric materials such as quartz, lithium niobate, lithium tantalate, or similar materials have been widely used for bulk wave devices such as piezoelectric transducers and elastic surface wave devices. In manufacturing these devices, wafer bonding technology and/or anodic bonding technology are employed to yield the desired compact size and yield.
With respect to wafer bonding technology, two substrates, either of the same or different material, are bonded without an intermediate adhesive layer such that adhesion by covalent bonding or ionic bonding between the atoms on the substrate surfaces occurs. Wafer bonding is accomplished by joining two mirror finished substrates and applying heat. anodic bonding is performed by joining two mirror finished substrates and heat treating them while, at the same time, applying a voltage to the interface between the two substrates. Bonding strength depends on the heat treatment temperature. Generally, the higher the heat treatment temperature, the stronger the bonding strength. However, when the treatment temperature becomes too high, adverse effects may occur. For example, if two substrates undergoing wafer bonding or anodic bonding have different thermal expansion rates, the substrates may break or delaminate due to the different thermal expansion coefficients.
Similar problems may occur with piezoelectric composite substrates or piezoelectric devices during manufacturing. For example, the average expansion coefficient of silicon from 25 to 300.degree. C. is 3.4.times.10.sup.-6 /.degree.C., while that of quartz is 15.2.times.10.sup.-6 /.degree.C., lithium niobate is 18.3.times.10.sup.-6 /.degree.C., and lithium tantalate is 19.9.times.10.sup.-6 /.degree.C. In these piezoelectric materials, the average expansion coefficients are in the x-axis direction of the crystals. Thus, the thermal expansion coefficient of quartz in the x direction is five times that of silicon, and this difference in thermal expansion coefficients may cause damage to the substrate combination.
Japanese laid-open patent Heisei-5-327383 discusses the relationship between the thickness of a quartz substrate and temperature where the substrate is damaged when a quartz substrate and a semiconductor substrate are bonded by wafer bonding. That is, it is reported that the thinner the quartz substrate, the higher the damage temperature, because the stress generated at the bonding portion is reduced. For example, for a large silicon substrate, the damage temperature at which the substrate is damaged is 350.degree. C. when the thickness of the quartz substrate is 80 .mu.m, while the damage temperature at which the substrate is damaged is 450.degree. C. when the thickness of the quartz substrate is 40 .mu.m. The temperature at which damage occurs varies according to the size and configuration of the substrate. Accordingly, the heat treatment temperatures when wafer bonding have to be lower than the temperature at which damage may occur.
Furthermore, single heat treatments may have adverse effects. That is, in wafer bonding, water structuring molecules exist at the bonding interface after initial joining. Thus, there are water structuring molecules at the bonding interface during the adhering step. While, most of the water structuring molecules are removed as the heat treatment temperature rises, some are trapped by surrounding adhesion. As a result, voids are created without adhesion. Thus, there exist at the bonding interface strongly bonded portions and void portions resulting in uneven distribution of thermal stresses. This condition may damage the substrates and cause delamination of the joined substrates.
In the case of ionic bonding, voids may be generated due to gas existing at the bonding interface either from the initial joining or from gas being generated during the heat treatment, thus resulting in the same problems as discussed above. Moreover, while gas can be generated during the bonding processes of the substrates, further heat treatments after the bonding process may also generate additional gas. For example, thermal stress from heating from solder reflowing may generate voids, may damage the substrates, and may cause delamination of the substrates.
To solve these problems according to conventional methods, wafer bonding is accomplished by employing two heat treatments and using thin substrates. That is, substrates are bonded temporarily at a relatively low first temperature, thinned by a mechanical method or chemical etching, and then bonded strongly at a relatively high second temperature to complete wafer bonding. See, for example, Japanese laid-open patents. H5-327383, H4-286310, H-3-97215. More particularly, substrates are bonded temporarily by heat treating at the first temperature at which damage of the substrates does not occur, then thinned by grinding, and finally bonded strongly at a temperature sufficiently high to obtain the desired bonding strength.
However, even these methods cannot prevent the problems of the aforementioned voids and delamination. Moreover, yield tends to become lower in actual manufacturing processes. Since removing water from a central portion of the substrate is difficult compared to the periphery of the substrate, it is difficult to prevent damage caused by the stress generated at the central portion of the substrate. Central water deposits become an ever bigger problem when substrate size is enlarged in order to reduce manufacturing costs.
Even if the damage or delamination of substrates can be avoided, the stress to the substrates may affect the performance characteristics of elements formed on the composite substrate due to insufficient bonding. More specifically, such imperfections affect the temperature-to-frequency characteristics of a piezoelectric device such as a piezoelectric vibrator or a piezoelectric filter. That is, thermal stress experienced by substrates is increased by temperature changes.
Stress caused by different thermal expansion coefficients may also change the crystal structure. When molecule structures are the same but various crystal structures exist, the crystal structures may be changed by pressure or temperature. For example, when quartz is heated up to more than 573.degree. C. under no pressure, the quartz transitions from .alpha.-quartz to .beta.-quartz. Other phase transitions besides .alpha.-.beta. phase transitions can also be seen. Dauphine twin type phase transitions which occur when stress is applied to quartz is one example. Moreover, a Dauphine twin type phase transition which occurs at high temperature is a nonreversible reaction as reported in Annual Symposium Frequency Control, Vol. 31, page 171 (1977). When an element is formed on a substrate on which these phase transitions have occurred, undesirable characteristics result. More specifically, in the case of a phase transition in AT cut quartz, dependency of frequency characteristics on temperature increases, thus stable operation of piezoelectric elements such as quartz vibrators and quartz filters cannot be realized.